Conventional fast pyrolysis of biomass entails rapid heating of a biomass feedstock in a hypoxic environment to produce a combination of non-condensable gases (C1-C4), condensable pyrolysis vapors and solid carbonaceous char. Conventional fast pyrolysis of biomass typically mixes biomass with a heated solid particles (i.e., “heat carrier”) to facilitate rapid heating of the biomass to a temperature ranging from 315° C. to 600° C. The resulting thermal-cracking of the heated feedstock produces non-condensable light gases, a solid carbonaceous char, and condensable pyrolysis vapors that can be converted to biofuels.
A major barrier to commercial implementation of this pyrolysis technology for production of biofuels is that the condensable pyrolysis vapors include many highly-reactive intermediate compounds comprising radicals. These compounds rapidly react to form secondary compounds that are difficult to upgrade to transportation fuels. One strategy to prevent this has been to minimize residence time of these primary pyrolysis vapors within the reactor to less than a few seconds, as increasing residence time directly correlates with an increase in undesirable products that negatively impact the yield of upgradable vapors. However, a total residence time of 45-60 sec or longer is often required to ensure complete pyrolysis of all cellulosic biomass components and to maximize vapor yields. Therefore, a short vapor residence time and long solids residence time are competing goals in a conventional biomass pyrolysis process.
Certain pyrolysis systems and processes have attempted to increase the yield of upgradeable pyrolysis vapors by minimizing the residence time of these vapors in the reactor. This has typically been achieved by either employing a small-volume pyrolysis reactor or increasing the throughput of a sweep gas to quickly move pyrolysis vapors out of the reactor. However, neither of these options is conducive to the design of a large, commercial-scale pyrolysis system. Small-volume pyrolysis reactors have a shorter vapor residence time that helps prevent secondary reactions of the pyrolysis vapors that can prevent subsequent upgrading. Unfortunately, small volume reactors often lack sufficient residence time to completely pyrolyze the feedstock, thereby lowering efficiency and yield beneath commercially-viable levels. Alternatively, increasing the throughput of sweep gas (or sweep gas rate) also decreases efficiency by: 1) excessively diluting the pyrolysis vapors, making subsequent catalytic upgrading less efficient, 2) requiring more energy to heat the larger volume of sweep gas, and 3) potentially increasing char entrainment in the pyrolysis vapors leaving the reactor.
A commercial scale process and system for biomass pyrolysis must increase the throughput of biomass while remaining efficient and maximizing the yield of upgradeable pyrolysis vapors. One conventional strategy for increasing biomass throughput is to employ a reactor comprising a mechanical device (e.g., an auger) that facilitates biomass movement through the reactor. Unfortunately, auger-type reactors become progressively less efficient as their size is increased to a commercial-scale throughput. For example, increasing the diameter or cross-section of an auger-type reactor leads to progressively less efficient mixing, and thus, less efficient heating of the feedstock. This decreases pyrolysis vapor yield. Alternatively, increasing the reactor length to achieve increased biomass residence time (thereby increasing pyrolysis vapor yield) simultaneously decreases efficiency by increasing the volume of heated sweep gas required to minimize pyrolysis vapor residence time (leading to the problems discussed above). The disadvantages of not providing sufficient biomass residence time include decreased yield (via incomplete pyrolysis) and the potential for clogging of the pyrolysis system due to continued pyrolysis of the biomass following its removal from the reactor.
Certainly, there is a need to improve fast pyrolysis processes and systems to allow the efficient pyrolysis of lignocellulosic biomass at commercial scale, and to facilitate commercial-scale rapid upgrading of biomass-derived pyrolysis vapors into products that are fungible with current petroleum-derived liquid hydrocarbon fuels.